It has been known for over 50 years that greater low frequency efficiency in a loudspeaker system may be obtained by incorporating a mass-compliance resonance device. There are two basic approaches in common use in connection with mass-compliance resonance devices in loudspeaker systems: the ducted port (sometimes referred to as a “vent”) and the passive radiator. Although the passive radiator has some advantages, the ducted port has generally been more popular because it is less expensive, easier to manufacture, and more compact than a passive radiator.
There are, however, disadvantages to the ducted port approach. An ideal ducted port would pass only low frequencies, reinforcing the low frequency output of an actively driven transducer, but adding no coloration or independent sonic signature above the ducted port's desired pass band. Acoustic disadvantages of ducted ports arise when a ducted port's performance deviates from this ideal, adding distortion (e.g., coloration and/or undesirable noise) to the mid- and/or high-frequency output of the loudspeaker system. These disadvantages tend to be more prominent at high air velocities within the ducted port. In addition, midrange frequencies generated by the back wave of an active driver can “leak” out of the ducted port, adding undesirable coloration to the loudspeaker's output.
It is well known to those skilled in the art that a vented loudspeaker system has a specific tuning frequency determined by the volume of air in the enclosure and the acoustic mass of air provided by the ducted port. As a rule, relatively low tuning frequencies are desirable for high performance loudspeaker systems. The tuning frequency of a vented loudspeaker system can be lowered by increasing the “acoustic mass” in the ducted port or by increasing compliance by increasing the enclosure volume.
The acoustic mass of a ducted port is directly related to the mass of air contained within the ducted port but inversely related to the cross-sectional area of the ducted port. This relationship suggests that to achieve a lower tuning frequency a longer ducted port with smaller cross-sectional area should be used. However a small cross-section is in conflict with the larger volume velocities of air required to reproduce higher sound pressure levels at lower frequencies. For example, if the diameter of a ducted port is too small or is otherwise improperly designed, non-linear behavior such as chuffing, whistling, or port-noise due to air turbulence can result in audible distortions and loss of efficiency at low frequencies particularly at higher levels of operation. In addition, viscous drag from air movement in the ducted port can result in additional loss of efficiency at lower frequencies.
One way to lower the velocity of air within a ducted port is to use a long and narrow cross-section. Ducted ports with long and narrow cross sections are often referred to as “slot ports.” As used herein, the term “slot port” refers to a port having a relatively narrow cross section at its exit, in which the cross-section exit ratio of the port exit's longer dimension to its shorter dimension is at least 4:1. Slot ports tend to have naturally lower air velocity than conventional round ports. However, slot ports tend to have higher port noise caused by turbulence, as they have more wall area for a given cross-section than a corresponding round port. Accordingly, front-loaded slot ports are rarely used in high-performance loudspeaker enclosures. Moreover, according to conventional wisdom, slot ports having a cross-section exit ratio of greater than 8:1 should be avoided altogether.
Increasing the cross-sectional area of a ducted port can also reduce turbulence and loss, but the length of the ducted port must be increased proportionally to maintain the proper acoustic mass for a given tuning frequency. However, increasing the cross-sectional area can also increase the amount of midrange leakage, and increasing the cross-sectional area also increases the amount of space that the port occupies on a loudspeaker's baffle and within the enclosure. Various formulas are typically used for determining a minimum standard cross section area for a cylindrical ducted port.
In some cases, the entrance and/or exit of a ducted port may be flared in order to reduce turbulent port noise. This approach can reduce port noise to a certain degree, but it also increases the size of the port exit on a speaker baffle. While large port exits are acceptable in some applications, large port exits can be difficult to implement in compact high performance loudspeaker systems, especially those intended for high-performance use in relatively small rooms.
U.S. Pat. No. 7,162,049 to Polk, Jr. discloses various means of controlling turbulence in a duct port by flaring the ends of the duct port. Similarly, U.S. Pat. No. 5,714,721 to Gawronski, et al discloses a port duct with a tapered cross section. However, both of these references require large port exits and may not be suitable for front-loaded use in a compact high-performance loudspeaker system.
Consequently, many loudspeaker designs rear-load the port, placing the port exit on the rear baffle of the loudspeaker enclosure. Rear-loading can decrease the audibility of turbulent port noise and midrange leakage compared to a front-loaded port. However, rear-loading the port also makes the loudspeaker system more sensitive to room placement, and it makes it virtually impossible to mount the loudspeaker system against a rear wall or to flush-mount the loudspeaker within a wall.